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Meetings & Talks Schedule 2008Go to: BCS Events Current Monday, January 7Control of attention and gaze in complex environmentsJelena Jovancevic, Brain and Cognitive Sciences (Advisor: Mary Hayhoe) CVS Thesis Defense: Meliora 366, 9:00-10:00 am Dealing with natural, complex scenes in everyday behavior, where one is surrounded by a variety of potentially relevant stimuli, poses an important problem for our visual system. Given the constraints set by attentional and working memory limitations in acquiring and retaining information, how does our visual system solve the problem of selecting appropriate information when it's needed, in the context of visually guided behavior? Though incomplete, overt fixations carry much information about current attentional state, and are a revealing indicator of this selection. What controls allocation of gaze and attention in natural environments? Traditionally, attention was thought to be attracted exogenously by the properties of the stimulus. Studies done using 2D experimental displays or viewing of scenes, showed that properties such as contrast or chromatic salience can explain some regularities in fixation patterns. These, however, can account for only a modest proportion of the variance. Further, experimental contexts examined may not reflect the challenges of natural visually guided behavior. Complexity of the environment and the ongoing behavior make it necessary to look at natural behavior when investigating control of gaze. Recent work in natural tasks has demonstrated that the observer's cognitive goals play a critical role in the distribution of gaze during ongoing natural behavior. The goal of this thesis is to understand the mechanisms that control the deployment of gaze in natural environments. Though fixation patterns in natural behavior are largely determined by the momentary task, it is not clear how effective top-down control is in dynamic environments because of the difficulty of dealing with unexpected events. To address this problem we studied gaze patterns in both real and virtual walking environments where subjects were occasionally exposed to potentially colliding pedestrians. Our results indicate potential collisions do not automatically attract attention and are usually detected by active search, rather than by reacting to looming. If, however, a collider is detected, fixations on all pedestrians are increased in the subsequent few seconds, indicating that subjects learn the structure and dynamic properties of the world in order to fixate critical regions at the right time. We also investigated whether an addition of another perceptually demanding task interferes with the detection of potential collisions. In a situation of walking, while also following a leader pedestrian, detection of colliders decreased significantly indicating that subject learn how to allocate attention and gaze to satisfy competing demands. In a real environment we investigated whether manipulation of the probability of a potential collision, of pedestrians with predetermined roles, is accompanied by a corresponding change in gaze allocation. We demonstrated that fixation patterns adjust very quickly to changes in the probabilistic structure of the environment that indicate different priorities for gaze allocation. Based on our results, it appears that observers learn to represent sufficient structure about the visual environment in order to guide eye movement in a pro-active manner, in anticipation of events that are likely to occur in the scene. To investigate the importance of behavioral relevance we compared fixation durations when walkers stopped instead of going on a collision path. Other than the reduction in fixation probabilities of about 20%, the pattern remained the same. This supports the idea that gaze behavior takes into account the risk (or reward) value of particular information, and is consistent with reinforcement learning models of gaze as well as with the neuro-physiological findings on the importance of reward. Finally we made a comparison of performance in real and virtual environments in order to evaluate the validity of the latter. The results in the virtual reality walking strengthen our result in the real walking experiment validating virtual environments as useful paradigms in the study of natural behavior. Thursday, January 10Probing attention and perception through high-resolution fMRI of the human subcortexKeith Schneider, RCBI CVS Research Talk: Kresge Rm., Meliora 269, 12:00-1:00 pm This is a practice job talk. The human visual system contains multiple streams of information that originate in distinct classes of retinal ganglion cells. These information streams remain disjoint in the subcortex, innervating for example the magnocellular and parvocellular layers of the lateral geniculate nucleus (LGN) and the superior colliculus, and become intermixed in the visual cortex. High resolution functional magnetic resonance imaging (fMRI) of the subcortex is the only way to directly access these information streams in humans, and can reveal significant structural detail. My collaborators and I have used fMRI to detect phenomenon in the subcortex, such as binocularly rivalry and until recently attention, that have not been observed here with primate electrophysiology. The subcortical nuclei generally reflect the properties of the visual cortex, but with different gains and tunings. For example, while the LGN exhibits a preference for attention to particular features, the superior colliculus does not but instead is activated during transitions between attended features. For spatial attention, both the LGN and superior colliculus are modulated, but the superior colliculus significantly more so. For both featural and spatial attention, the activity of the pulvinar nuclei are intermediate to the LGN and superior colliculus. Examining the subcortical nuclei is an important step in understanding the function and architecture of the visual system. In addition, I will discuss clinical applications of this research for understanding dyslexia and congenital stationary night blindness. Monday, January 14Variation, signal, and noise in sensory-motor processing for smooth pursuit eye movementsStephen Lisberger, University of California San Francisco CVS Boynton Colloquium: Kresge Rm., Meliora 269, 12:00-1:30 pm Co-sponsored by Neurobiology & Anatomy/Neuroscience How are sensory inputs represented, decoded, and transformed into commands for movement? I will address this general issue by analysis of trial-by-trial variation in neural and behavioral measures during smooth pursuit eye movements. I will present evidence that most of the variation in the initiation of pursuit arises from noise in sensory representations. I also will show how the sensory-motor system can be thought of in terms of noise reduction and noise addition at each level of processing, and how this approach can be used to understand the transformation from vision to action. Friday, January 18Perceiving three dimensional space: slant and distanceAlen Hajnal, University of Louisville CVS Research Talk: Kresge Rm., Meliora 269, 12:00-1:00 pm Surface slant and distance are two properties of space with direct relevance to action. In two experiments I demonstrate that visual space in the dark has an intrinsic bias that represents the horizontal ground plane as slanted. Furthermore, the symmetry of the visual space around eye-level is preserved in the accuracy of perceived spatial locations, but slightly compromised in terms of the variability of perceptual judgments. The intrinsic bias of the visual system is present in full cue conditions as well, as exhibited by various explicit and implicit measures of slant perception. Implications for the perception and actualization of possibilities for action (affordances) are discussed. Monday, January 21Perceptual organization of contoursJames Elder, York University CVS Boynton Colloquium: Kresge Rm., Meliora 269, 12:00-1:30 pm Humans have a remarkable ability to rapidly group and organize image data into coherent representations reflecting the structure of the visual scene. However current computer vision algorithms are by comparison relatively primitive in their performance. Key issues include the combinatorial complexity of the problem and difficulties capturing and combining global constraints with local cues. In this work we develop a coarse-to-fine Bayesian algorithm that addresses these issues. In our approach, candidate contours are extracted at a coarse scale and then used to generate spatial priors on the location of possible contours at finer scales. In this way, a rough estimate of the shape of an object is progressively refined. The coarse estimate provides robustness to texture and clutter while the refinement process allows for the extraction of detailed shape information. The grouping algorithm is probabilistic and uses multiple grouping cues derived from natural scene statistics. We present a quantitative evaluation of grouping performance on natural images and show that the multi-scale approach outperforms single-scale contour extraction algorithms. We suggest that the substantial feedback connections known to exist in ventral stream of the visual cortex may support an analogous refinement of perceptual representations in the human brain. Wednesday, January 30The effects of action video game experience on perceptual decision makingC. Shawn Green, Brain & Cognitive Sciences (Advisor: Daphne Bavelier) CVS Thesis Defense: Kresge Rm., Meliora 269, 2:00-3:00 pm Action video game players (VGPs) have been shown to outperform their non-game playing (NVGPs) peers on a number of sensory/cognitive measures. In tasks that require accurate responses to quickly presented visual stimuli, VGPs typically exhibit higher levels of accuracy than NVGPs. In particular, VGPs have demonstrated enhancements in a number of tasks thought to tap reasonably independent aspects of visual attention (spatial distribution and resolution, temporal characteristics, capacity, etc). In tasks that require speeded responses, the VGP enhancement is observed as a large decrease in reaction time (RT) in VGPs compared to NVGPs (accuracy is typically equivalent in the two groups). Here we put forward the hypothesis that a single mechanistic explanation, an increase in the rate of sensory integration in VGPs, can account for the entirety of the data, thus bridging the gap between the accuracy and RT literatures. To test this hypothesis, two sensory integration tasks were employed - a standard motion coherence paradigm and a novel auditory localization task which, in combination with a model developed by Palmer et al (2005), allow for a more explicit test of the relative contribution of sensory integration rate, criteria, and motor execution in generating the differences observed between VGPs and NVGPs. In both the motion and auditory tasks, VGPs demonstrated a large reduction in RT compared to NVGPs with equivalent accuracy. This pattern was well captured by the model with an increase in the rate of information accrual and a concurrent decrease in criteria in the VGPs. Several follow-up experiments provide further support for the hypothesis that VGPs acquire sensory information more rapidly than NVGPs. Importantly, similar effects can be induced in NVGPs through extensive action video game training. Finally, to examine how these changes may be implemented at the neural level, a model by Ma and colleagues (2007) was utilized, with the primary difference between VGPs and NVGPs being an increase in the strength of feed-forward projections between sensory and integration areas in the VGP group. Monday, February 4Synchronous cortical oscillations alter the stability of spatial activity patterns in primary visual cortexNathan Rosecrans, Center for Visual Science CVS Research Talk: Kresge Rm., Meliora 269, 12:00-1:00 pm Sensory coding in the primary visual cortex is understood to occur primarily through the selective activation of neurons that encode specific local features of the visual environment. However, during visual stimulation, visual cortical activity exhibits strong spatial and temporal correlations that are not derived from the visual input. Rather, these correlations reflect coordinated activity that is produced spontaneously by the cortical network, which have been shown to strongly shape cortical responses to sensory stimuli. In awake animals the fine-scale spatio-temporal structure of this activity remains unclear. Using a 3.2mm linear array of 16 multi-unit electrodes, we examined spontaneous neural activity within the primary visual cortex of awake ferrets in complete darkness. We found spatial patterns of neural firing that reflected coordinated activity across neighboring cortical columns. During the majority of spontaneous activity, these local regions of elevated firing fluctuated rapidly, persisting from between 50ms to 100ms. Cortical activity during this dynamic behavior was dominated by synchronous alpha (8-12Hz) band oscillations. Interspersed within these periods, were episodes in which the local patterns of activity became stabilized within specific cortical columns, which lasted from 500ms up to 5sec. In contrast to the periods of spatial pattern instability, these periods of stabilization were marked by little power or synchrony within the alpha band, and increased firing rate and power in the gamma (26-80Hz) band. These findings establish a link between the large-scale oscillatory behavior of cortical networks and the spatial distribution of activity within local cortical columns. Monday, February 18Where do correlations between neuronal activity and sensory decisions originate?Bruce Cumming, National Eye Institute CVS Boynton Colloquium: Kresge Rm., Meliora 269, 12:00-1:30 pm Simultaneous use of single unit recording and threshold psychophysics has revealed correlations between perceptual choice and firing rate, that cannot be explained by the visual stimulus (Choice Probability, CP). Quantitative modeling studies have explained the observed magnitudes with a bottom-up scheme, in which CP reflect an effect of random fluctuations in firing rate upon choice. In order to test this interpretation further, we measured CP using a stimulus which simultaneously allowed the use of white noise analysis to infer how fluctuations in the stimulus content affected neuronal activity. Two monkeys performed a disparity identification task while we recorded the activity of disparity selective neurons in V2. The stimulus was a random dot stereogram in which the disparity was chosen at random (from a discrete distribution) for each 10ms video frame. Signal was added by increasing the probability with which one disparity was presented on a given frame. Calculating the mean response following one video frame, for each disparity, yields disparity response functions. These were calculated separately according to the choice reported at the end of the trial. Trials (with no added signal) on which animals report the preferred disparity have higher mean firing rates - as expected from earlier observations on CP. The disparity response functions reveal that this mainly reflects an increase in the gain of the neuronal response to disparity, on trials where the animal chooses that neurons preferred disparity. We have been unable to generate such large gain changes in (bottom-up) simulations where the pooled response of a neuronal population determines choice. These gain changes resemble the effects of spatial or feature-based attention that have been reported by others. This suggests that a significant component of the CP in this task reflects a top-down process similar to feature-based attention. Monday, March 31Comparing structural and functional damage due to glaucomaDonald Hood, Columbia University CVS Boynton Colloquium: Kresge Rm., Meliora 269, 12:00-1:30 pm Glaucoma, the leading cause of preventable blindness, produces a loss of vision by damaging retinal ganglion cell (RGC) axons. What is the relationship between the loss of vision and the loss of RGCs? It is commonly believed that structural damage (e.g. loss of RGC axons) precedes functional damage (i.e. loss of behavioral sensitivity). The loss of RGC axons can now be measured in vivo with optical coherence tomography (OCT). Data from patients and controls indicate that behavioral and structural damage precede at the same rate. In fact, a simple linear model relates the local loss of RGC axons, as measured with OCT, to local losses in behavioral sensitivity. Structural damage appears to precede functional damage only under conditions where the measurement of structural damage is less variable than the measurement of the functional damage. Monday, April 14What information do we use to detect and identify natural images?Peter Bex, Harvard University CVS Research Talk: Kresge Rm., Meliora 269, 12:00-1:00 pm Much of our understanding of visual processing comes from experiments involving foveally-viewed, isolated sinusoidal gratings that are presented briefly at barely-visible contrasts. Compared with such laboratory conditions, natural vision is concerned with complex images that contain broad distributions of spatio-temporal structure. I describe a range of psychophysical and analytical techniques that examine the information in natural scenes that is selected by the human visual system to guide behaviourally-relevant decisions. Friday, April 18In vivo imaging of the retinal pigment epithelial cellsJessica Morgan, Optics CVS Thesis Defense: Goergen 101, 10:00-11:00 am The retinal pigment epithelial (RPE) cells are an important layer of the retina because they are responsible for providing metabolic support to the photoreceptors. Techniques to image the RPE layer include autofluorescence imaging with a scanning laser ophthalmoscope (SLO). However, previous studies were unable to resolve single RPE cells. This thesis describes the technique of combining autofluorescence, SLO, adaptive optics (AO), and dual wavelength simultaneous imaging and registration to visualize the individual cells in the RPE mosaic in human and primate retina for the first time in vivo. After imaging the RPE mosaic non-invasively, the cell layer's structure and regularity were characterized using quantitative metrics of cell density, spacing, and nearest neighbor distances. The RPE mosaic was compared to the cone mosaic, and RPE imaging methods were confirmed using histology. The ability to image the RPE mosaic led to the discovery of a novel retinal change following light exposure; 568 nm exposures caused an immediate reduction in autofluorescence followed by either full recovery or permanent damage in the RPE layer. A safety study was conducted to determine the range of exposure irradiances that caused permanent damage or transient autofluorescence reductions. Additionally, the threshold exposure causing autofluorescence reduction was determined and reciprocity of radiant exposure was confirmed. Light exposures delivered by the AOSLO were not significantly different to those delivered by a uniform source. As all exposures tested were near or below the permissible light levels of safety standards, this thesis provides evidence that the current light safety standards need to be revised. Finally, with the retinal damage and autofluorescence reduction thresholds, the methods of RPE imaging were modified to allow successful imaging of the individual cells in the RPE mosaic while still ensuring retinal safety. This thesis has provided a highly sensitive method for studying the in vivo morphology of individual RPE cells in normal, diseased, and damaged retinas. The methods presented here also will allow longitudinal studies for tracking disease progression and assessing treatment efficacy in human patients and animal models of retinal diseases affecting the RPE. Monday, April 21From fragments to objects: mechanisms of visual integrationPawan Sinha, Massachusetts Institute of Technology CVS Boynton Colloquium: Kresge Rm., Meliora 269, 12:00-1:30 pm Learning to integrate information is a key task a child's brain has to perform during the normal developmental course. In essence, this developmental process transforms the sensorium from an amorphous collection of primitive attributes, to one where these attributes are integrated into cliques corresponding to distinct objects. In order to study this process experimentally, we have recently launched Project Prakash--a humanitarian/scientific initiative that helps provide sight to children suffering from treatable congenital blindness, and characterizes their subsequent visual development. In this talk, I shall describe a case study from this project, and also outline a computational model motivated by the experimental results. Monday, May 19Learning for adaptive decisions in humans, brains, and machinesZoe Kourtzi, University of Birmingham CVS Boynton Colloquium: Kresge Rm., Meliora 269, 12:00-1:30 pm Successful actions and interactions in the complex environments we inhabit entail making fast and optimal perceptual decisions. Extracting the key features from our sensory experiences, assigning them to meaningful categories and deciding how to interpret them is a computationally challenging task that is far from understood. Accumulating evidence suggests that the brain is optimized to solve this challenge by combining sensory information and previous knowledge about the environment acquired through evolution, development and everyday experience. We combine psychophysics, fMRI and advanced mathematical approaches to investigate the neural mechanisms that mediate experience-dependent plasticity in the human brain. Our studies show that the human brain learns to exploit regularities in the environment and flexible rules of organization of the physical input. Our findings suggest that experience plays an important role in the adaptive optimization of visual functions by shaping neural processing across cortical networks in the human brain. Monday, May 19Seeing in depth: convergence and combinationAndrew Welchman, University of Birmingham CVS Research Talk: Kresge Rm., Meliora 269, 4:00-5:00 pm Estimating the depth structure of the environment is a principal function of the visual system, enabling many key computations, such as segmentation, object recognition, material perception and the guidance of movements. The brain exploits a range of depth cues to estimate depth, combining information from shading and shadows to linear perspective, motion and binocular disparity. Despite the importance of this process, we still know relatively little about the cortical processing of depth cues and their synthesis. Our recent fMRI work aims to understand the functional role of different cortical areas in the processing of perceptually-useful depth information. I will suggest an alternative view of the functional roles of the dorsal and ventral streams in the processing of depth information, based on the idea that they perform computations with different goals. Friday, May 23Optical radiation hazards to the eyeDavid Sliney CVS Research Talk: Kresge Rm., Meliora 269, 12:00-1:00 pm Humans have evolved under a constant bath of "dangerous" ultraviolet rays and if they stare at the sun, they can experience a permanent retinal injury--an eclipse blindness. Nevertheless, ocular tissues are surprsingly well protected by anatomical, physiological and behavioral factors. Sunglasses may actually increase environmental risks to the eye, and ophthalmic instrument light may also pose serious risk of injury. Exposure limits for cornea, lens and retina have been developed to protect the eye from artificial light sources and lasers, but it is important to recognize the underlying assumptions that form the basis of these exposure limits, and when adjustments are required--as in the case of ophthalmic instruments. No June talks scheduled. No July talks scheduled. No August talks scheduled. No September talks scheduled. No October talks scheduled. No November talks scheduled. No December talks scheduled. |